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  • richardmitnick 12:23 pm on September 12, 2014 Permalink | Reply
    Tags: , , Quantum Mechanics   

    From M.I.T.: “Fluid mechanics suggests alternative to quantum orthodoxy” 


    MIT News

    September 12, 2014
    Larry Hardesty | MIT News Office

    New math explains dynamics of fluid systems that mimic many peculiarities of quantum mechanics.

    The central mystery of quantum mechanics is that small chunks of matter sometimes seem to behave like particles, sometimes like waves. For most of the past century, the prevailing explanation of this conundrum has been what’s called the “Copenhagen interpretation” — which holds that, in some sense, a single particle really is a wave, smeared out across the universe, that collapses into a determinate location only when observed.

    wave
    Close-ups of an experiment conducted by John Bush and his student Daniel Harris, in which a bouncing droplet of fluid was propelled across a fluid bath by waves it generated. Image: Dan Harris

    But some founders of quantum physics — notably Louis de Broglie — championed an alternative interpretation, known as “pilot-wave theory,” which posits that quantum particles are borne along on some type of wave. According to pilot-wave theory, the particles have definite trajectories, but because of the pilot wave’s influence, they still exhibit wavelike statistics.

    John Bush, a professor of applied mathematics at MIT, believes that pilot-wave theory deserves a second look. That’s because Yves Couder, Emmanuel Fort, and colleagues at the University of Paris Diderot have recently discovered a macroscopic pilot-wave system whose statistical behavior, in certain circumstances, recalls that of quantum systems.

    Couder and Fort’s system consists of a bath of fluid vibrating at a rate just below the threshold at which waves would start to form on its surface. A droplet of the same fluid is released above the bath; where it strikes the surface, it causes waves to radiate outward. The droplet then begins moving across the bath, propelled by the very waves it creates.

    “This system is undoubtedly quantitatively different from quantum mechanics,” Bush says. “It’s also qualitatively different: There are some features of quantum mechanics that we can’t capture, some features of this system that we know aren’t present in quantum mechanics. But are they philosophically distinct?”

    Tracking trajectories

    Bush believes that the Copenhagen interpretation sidesteps the technical challenge of calculating particles’ trajectories by denying that they exist. “The key question is whether a real quantum dynamics, of the general form suggested by de Broglie and the walking drops, might underlie quantum statistics,” he says. “While undoubtedly complex, it would replace the philosophical vagaries of quantum mechanics with a concrete dynamical theory.”

    Last year, Bush and one of his students — Jan Molacek, now at the Max Planck Institute for Dynamics and Self-Organization — did for their system what the quantum pioneers couldn’t do for theirs: They derived an equation relating the dynamics of the pilot waves to the particles’ trajectories.

    In their work, Bush and Molacek had two advantages over the quantum pioneers, Bush says. First, in the fluidic system, both the bouncing droplet and its guiding wave are plainly visible. If the droplet passes through a slit in a barrier — as it does in the re-creation of a canonical quantum experiment — the researchers can accurately determine its location. The only way to perform a measurement on an atomic-scale particle is to strike it with another particle, which changes its velocity.

    The second advantage is the relatively recent development of chaos theory. Pioneered by MIT’s Edward Lorenz in the 1960s, chaos theory holds that many macroscopic physical systems are so sensitive to initial conditions that, even though they can be described by a deterministic theory, they evolve in unpredictable ways. A weather-system model, for instance, might yield entirely different results if the wind speed at a particular location at a particular time is 10.01 mph or 10.02 mph.

    The fluidic pilot-wave system is also chaotic. It’s impossible to measure a bouncing droplet’s position accurately enough to predict its trajectory very far into the future. But in a recent series of papers, Bush, MIT professor of applied mathematics Ruben Rosales, and graduate students Anand Oza and Dan Harris applied their pilot-wave theory to show how chaotic pilot-wave dynamics leads to the quantumlike statistics observed in their experiments.

    What’s real?

    In a review article appearing in the Annual Review of Fluid Mechanics, Bush explores the connection between Couder’s fluidic system and the quantum pilot-wave theories proposed by de Broglie and others.

    The Copenhagen interpretation is essentially the assertion that in the quantum realm, there is no description deeper than the statistical one. When a measurement is made on a quantum particle, and the wave form collapses, the determinate state that the particle assumes is totally random. According to the Copenhagen interpretation, the statistics don’t just describe the reality; they are the reality.

    But despite the ascendancy of the Copenhagen interpretation, the intuition that physical objects, no matter how small, can be in only one location at a time has been difficult for physicists to shake. Albert Einstein, who famously doubted that God plays dice with the universe, worked for a time on what he called a “ghost wave” theory of quantum mechanics, thought to be an elaboration of de Broglie’s theory. In his 1976 Nobel Prize lecture, Murray Gell-Mann declared that Niels Bohr, the chief exponent of the Copenhagen interpretation, “brainwashed an entire generation of physicists into believing that the problem had been solved.” John Bell, the Irish physicist whose famous theorem is often mistakenly taken to repudiate all “hidden-variable” accounts of quantum mechanics, was, in fact, himself a proponent of pilot-wave theory. “It is a great mystery to me that it was so soundly ignored,” he said.

    Then there’s David Griffiths, a physicist whose Introduction to Quantum Mechanics is standard in the field. In that book’s afterword, Griffiths says that the Copenhagen interpretation “has stood the test of time and emerged unscathed from every experimental challenge.” Nonetheless, he concludes, “It is entirely possible that future generations will look back, from the vantage point of a more sophisticated theory, and wonder how we could have been so gullible.”

    “The work of Yves Couder and the related work of John Bush … provides the possibility of understanding previously incomprehensible quantum phenomena, involving ‘wave-particle duality,’ in purely classical terms,” says Keith Moffatt, a professor emeritus of mathematical physics at Cambridge University. “I think the work is brilliant, one of the most exciting developments in fluid mechanics of the current century.”

    See the full article here.

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  • richardmitnick 4:15 pm on September 4, 2014 Permalink | Reply
    Tags: , , Quantum Mechanics   

    From PBS NOVA: “Quantum Physicists Catch a Pilot Wave” 

    PBS NOVA

    NOVA

    Wed, 03 Sep 2014
    Jennifer Ouellette

    In October 1927, some of the greatest minds in physics gathered for the Fifth Solvay International Conference to debate the troubling implications of the then-nascent theory of quantum mechanics. A particularly contentious topic was the perplexing “wave-particle duality,” in which objects we typically think of as particles—like photons and electrons—exhibit wave-like properties as well, and things we think of as waves, like light, sometimes behave like particles.

    The French physicist Louis de Broglie proposed a means by which a photon or electron could behave like both a particle and a wave, complementary aspects of the same phenomenon. He reasoned that the particles could be carried along by what he dubbed “pilot waves”—fluid-like ripples in space and time—much like a buoys bobbing along with the tide.

    droplet

    De Broglie won the Nobel Prize in Physics just two years later, but it wasn’t for pilot waves. His contemporaries largely dismissed his explanation for the dual nature of subatomic particles. Now, more than 80 years later, a series of experiments on the behavior of oil droplets bouncing along a vibrating liquid surface have provided a macroscale analog of de Broglie’s pilot waves, replicating some of the stranger properties of quantum mechanics.

    Quantum mechanics seeks to describe nature at the level of individual atoms and the particles that comprise them. But when physicists began delving into this strange new realm at the dawn of the 20th century, they discovered that the old, deterministic laws of classical physics no longer apply at that scale. Instead, uncertainty reigns supreme. It is a world governed by probabilities, and many physicists found this disquieting, to say the least. Hence Albert Einstein’s famous declaration at the Solvay conference that God does not play dice with the universe, prompting Niels Bohr to counter, “Einstein, stop telling God what to do.”

    At the heart of the discomfort is the question of uncertainty. Flip a coin, and it will land either heads or tails; in principle, with complete information about the coin, the hand doing the flipping, and the movement of air molecules around the flip, it is possible to predict the outcome. In the quantum world, things hover in a fuzzy, nebulous cloud of probability called a wave function that encompasses all potential states, with no prospect of gaining further information. Flip a quantum coin, and it is both heads and tails until we look. Things become definite only when an observation forces them to settle on a specific outcome.

    To Einstein, the notion of observation dictating the outcome of an experiment was ridiculous, since it denied the existence of a solid underlying reality. Even [Erwin]Schrödinger, inventor of the wave function, was deeply disturbed by the implications of what he’d helped create, memorably declaring, “I don’t like it, and I’m sorry I ever had anything to do with it.”

    De Broglie’s alternative pilot wave theory was an attempt to restore that underlying solid reality. Instead of the wave function, de Broglie’s pilot wave theory employs two equations, one describing an actual wave and the other describing the path of an actual particle and how it interacts with, and is guided by, the wave equation. It is deterministic, like a classical coin flip. In principle, at least, we can glean sufficient information to plot a particle’s path, something that is not allowed in Bohr’s interpretation of quantum mechanics.

    While the idea of pilot wave theory never really caught on, it stubbornly refused to die. A physicist named David Bohm proposed a modified version in the 1950s that also failed to gain much traction. But perhaps the pilot wave’s time has come at last.

    The latest resurgence of interest began in Paris about ten years ago, when Yves Couder and Emmanuel Fort of Diderot University started experimenting with oil droplets bounced off a vat of vibrating liquid. The droplet’s impact causes waves to ripple outward, like tossing a pebble in a pond. If the liquid in the vat vibrates at just the right frequency, usually quite close to the droplet’s natural resonant frequency, the droplet interacts with the ripples it creates as it bounces along, which in turn can affect its path. That’s eerily similar to de Broglie’s notion of a pilot wave. Such a system turns out to be a fantastic means for simulating weird quantum effects like the dual nature of light and matter.

    The 19th century physicist Thomas Young demonstrated this with his famous double-slit experiment. In the double slit experiment, a series of photons or electrons strike a screen with two slits in it before landing on a detector behind the screen. If you consider photons and electrons to be particles, you would expect the detector light up along the path through the slits and nowhere else. But that’s not what Young found. Instead, Young discovered an interference pattern of alternating light and dark bands, suggesting that the would-be particles were acting like water waves passing through a barrier wall with two openings. But if one places detectors near the slits to “see” which slit each particle went through, then the interference pattern disappears—the waves start acting like particles. It is the essence of quantum weirdness.

    Couder and Fort replicated Young’s experiment by steering their bouncing droplets toward such a screen with a slit, helped along by the pilot waves created by the vibrating liquid. While they appear to scatter randomly as they pass through the slit, over time, wavy interference patterns emerge. “Guided” by the pilot waves, the droplets appear to be drawn to those regions where the wavefronts add together, and steer clear of those regions where the wavefronts cancel each other out. Disturbing the pilot wave destroys the interference pattern, much like measuring the path of particles as they hit the screen does.

    Last year, pilot wave theory received another boost when MIT physicists Daniel Harris and John Bush used a similar fluid system to mimic a “quantum corral,” in which electrons are trapped within a ring of ions. Harris and Bush made a shallow tray with a circle-shaped trough in the center to serve as the walls of the “corral.” They filled the tray with silicon oil and mounted it on a vibrating stand tuned to a frequency just shy of that required to produce pilot waves spontaneously, without the droplet, according to Bush. Above that threshold, the roiling sea of waves will interfere with the droplet’s walk. Below it, the surface remains smooth except for the waves produced by the bouncing droplet. The closer one tunes the vibrations to that threshold, the more robust and long-lived the generated pilot waves will be.

    When the bouncing droplet produced waves, those waves bounced off the walls and interfered with each other, producing pretty interference patterns. They also affected the trajectory of the droplet. At first it looked like it was bouncing along randomly, but over time (around 20 minutes), the droplet was far more likely to drift towards the center of the circle, and increasingly less likely to be found in the rippling rings spreading out from that center. That probability distribution for the single droplet proved very similar to that of an electron trapped in a quantum corral.

    The droplet experiments provide an intriguing analogue or “toy model” for de Broglie’s pilot waves, but there is still no direct evidence of pilot waves at the quantum scale. “Time will tell whether the quantum-like behavior of the walking dropets is mere coincidence,” Bush told me via email. Also, the theory is currently limited to describing the simplest interactions between particles and electromagnetic fields. “It is not by itself capable of representing very much physics,” Oxford University physics philosopher David Wallace told Quanta earlier this year. “In my own view, this is the most severe problem for the theory, though, to be fair, it remains an active research area.”

    Nobody is claiming that quantum mechanics is wrong; there is too much experimental evidence that the equations do make accurate predictions about how things work at the subatomic scale. But the implications of the standard interpretations remain troubling. The pioneers of quantum mechanics came up with the most plausible theory they could, given the resources they had, and they transformed modern physics in the process. Contemplating the possibility of pilot wave theory might lead to a fresh interpretation of quantum weirdness, one that prompts physicists to rethink their longstanding assumptions about the true nature of the quantum world. Another transformation could be lurking in the wings.

    See the full article, with video, here.

    NOVA is the highest rated science series on television and the most watched documentary series on public television. It is also one of television’s most acclaimed series, having won every major television award, most of them many times over.

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  • richardmitnick 2:21 pm on August 19, 2014 Permalink | Reply
    Tags: , , Quantum Mechanics, ,   

    From Quanta: “At Multiverse Impasse, a New Theory of Scale” 

    Quanta Magazine
    Quanta Magazine

    August 18, 2014
    Natalie Wolchover

    Mass and length may not be fundamental properties of nature, according to new ideas bubbling out of the multiverse.

    Though galaxies look larger than atoms and elephants appear to outweigh ants, some physicists have begun to suspect that size differences are illusory. Perhaps the fundamental description of the universe does not include the concepts of “mass” and “length,” implying that at its core, nature lacks a sense of scale.

    This little-explored idea, known as scale symmetry, constitutes a radical departure from long-standing assumptions about how elementary particles acquire their properties. But it has recently emerged as a common theme of numerous talks and papers by respected particle physicists. With their field stuck at a nasty impasse, the researchers have returned to the master equations that describe the known particles and their interactions, and are asking: What happens when you erase the terms in the equations having to do with mass and length?

    Nature, at the deepest level, may not differentiate between scales. With scale symmetry, physicists start with a basic equation that sets forth a massless collection of particles, each a unique confluence of characteristics such as whether it is matter or antimatter and has positive or negative electric charge. As these particles attract and repel one another and the effects of their interactions cascade like dominoes through the calculations, scale symmetry “breaks,” and masses and lengths spontaneously arise.

    Similar dynamical effects generate 99 percent of the mass in the visible universe. Protons and neutrons are amalgams — each one a trio of lightweight elementary particles called quarks. The energy used to hold these quarks together gives them a combined mass that is around 100 times more than the sum of the parts. “Most of the mass that we see is generated in this way, so we are interested in seeing if it’s possible to generate all mass in this way,” said Alberto Salvio, a particle physicist at the Autonomous University of Madrid and the co-author of a recent paper on a scale-symmetric theory of nature.

    In the equations of the “Standard Model of particle physics”, only a particle discovered in 2012, called the Higgs boson, comes equipped with mass from the get-go. According to a theory developed 50 years ago by the British physicist Peter Higgs and associates, it doles out mass to other elementary particles through its interactions with them. Electrons, W and Z bosons, individual quarks and so on: All their masses are believed to derive from the Higgs boson — and, in a feedback effect, they simultaneously dial the Higgs mass up or down, too.

    sm
    The Standard Model of elementary particles, with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    The new scale symmetry approach rewrites the beginning of that story.

    as
    Alessandro Strumia of the University of Pisa, pictured speaking at a conference in 2013, has co-developed a scale-symmetric theory of particle physics called “agravity.” Thomas Lin/Quanta Magazine

    “The idea is that maybe even the Higgs mass is not really there,” said Alessandro Strumia, a particle physicist at the University of Pisa in Italy. “It can be understood with some dynamics.”

    The concept seems far-fetched, but it is garnering interest at a time of widespread soul-searching in the field. When the Large Hadron Collider at CERN Laboratory in Geneva closed down for upgrades in early 2013, its collisions had failed to yield any of dozens of particles that many theorists had included in their equations for more than 30 years. The grand flop suggests that researchers may have taken a wrong turn decades ago in their understanding of how to calculate the masses of particles.

    “We’re not in a position where we can afford to be particularly arrogant about our understanding of what the laws of nature must look like,” said Michael Dine, a professor of physics at the University of California, Santa Cruz, who has been following the new work on scale symmetry. “Things that I might have been skeptical about before, I’m willing to entertain.”

    The Giant Higgs Problem

    The scale symmetry approach traces back to 1995, when William Bardeen, a theoretical physicist at Fermi National Accelerator Laboratory in Batavia, Ill., showed that the mass of the Higgs boson and the other Standard Model particles could be calculated as consequences of spontaneous scale-symmetry breaking. But at the time, Bardeen’s approach failed to catch on. The delicate balance of his calculations seemed easy to spoil when researchers attempted to incorporate new, undiscovered particles, like those that have been posited to explain the mysteries of dark matter and gravity.

    Instead, researchers gravitated toward another approach called “supersymmetry” that naturally predicted dozens of new particles. One or more of these particles could account for dark matter. And supersymmetry also provided a straightforward solution to a bookkeeping problem that has bedeviled researchers since the early days of the Standard Model.

    Supersymmetry standard model
    Standard Model of Supersymmetry

    In the standard approach to doing calculations, the Higgs boson’s interactions with other particles tend to elevate its mass toward the highest scales present in the equations, dragging the other particle masses up with it. “Quantum mechanics tries to make everybody democratic,” explained theoretical physicist Joe Lykken, deputy director of Fermilab and a collaborator of Bardeen’s. “Particles will even each other out through quantum mechanical effects.”

    This democratic tendency wouldn’t matter if the Standard Model particles were the end of the story. But physicists surmise that far beyond the Standard Model, at a scale about a billion billion times heavier known as the “Planck mass,” there exist unknown giants associated with gravity. These heavyweights would be expected to fatten up the Higgs boson — a process that would pull the mass of every other elementary particle up to the Planck scale. This hasn’t happened; instead, an unnatural hierarchy seems to separate the lightweight Standard Model particles and the Planck mass.

    With his scale symmetry approach, Bardeen calculated the Standard Model masses in a novel way that did not involve them smearing toward the highest scales. From his perspective, the lightweight Higgs seemed perfectly natural. Still, it wasn’t clear how he could incorporate Planck-scale gravitational effects into his calculations.

    Meanwhile, supersymmetry used standard mathematical techniques, and dealt with the hierarchy between the Standard Model and the Planck scale directly. Supersymmetry posits the existence of a missing twin particle for every particle found in nature. If for each particle the Higgs boson encounters (such as an electron) it also meets that particle’s slightly heavier twin (the hypothetical “selectron”), the combined effects would nearly cancel out, preventing the Higgs mass from ballooning toward the highest scales. Like the physical equivalent of x + (–x) ≈ 0, supersymmetry would protect the small but non-zero mass of the Higgs boson. The theory seemed like the perfect missing ingredient to explain the masses of the Standard Model — so perfect that without it, some theorists say the universe simply doesn’t make sense.

    Yet decades after their prediction, none of the supersymmetric particles have been found. “That’s what the Large Hadron Collider has been looking for, but it hasn’t seen anything,” said Savas Dimopoulos, a professor of particle physics at Stanford University who helped develop the supersymmetry hypothesis in the early 1980s. “Somehow, the Higgs is not protected.”

    The LHC will continue probing for convoluted versions of supersymmetry when it switches back on next year, but many physicists have grown increasingly convinced that the theory has failed. Just last month at the International Conference of High-Energy Physics in Valencia, Spain, researchers analyzing the largest data set yet from the LHC found no evidence of supersymmetric particles. (The data also strongly disfavors an alternative proposal called “technicolor.”)

    The multiverse hypothesis has surged in begrudging popularity in recent years. But the argument feels like a cop-out to many, or at least a huge letdown.

    The implications are enormous. Without supersymmetry, the Higgs boson mass seems as if it is reduced not by mirror-image effects but by random and improbable cancellations between unrelated numbers — essentially, the initial mass of the Higgs seems to exactly counterbalance the huge contributions to its mass from gluons, quarks, gravitational states and all the rest. And if the universe is improbable, then many physicists argue that it must be one universe of many: just a rare bubble in an endless, foaming “multiverse.” We observe this particular bubble, the reasoning goes, not because its properties make sense, but because its peculiar Higgs boson is conducive to the formation of atoms and, thus, the rise of life. More typical bubbles, with their Planck-size Higgs bosons, are uninhabitable.

    “It’s not a very satisfying explanation, but there’s not a lot out there,” Dine said.

    As the logical conclusion of prevailing assumptions, the multiverse hypothesis has surged in begrudging popularity in recent years. But the argument feels like a cop-out to many, or at least a huge letdown. A universe shaped by chance cancellations eludes understanding, and the existence of unreachable, alien universes may be impossible to prove. “And it’s pretty unsatisfactory to use the multiverse hypothesis to explain only things we don’t understand,” said Graham Ross, an emeritus professor of theoretical physics at the University of Oxford.

    The multiverse ennui can’t last forever.

    “People are forced to adjust,” said Manfred Lindner, a professor of physics and director of the Max Planck Institute for Nuclear Physics in Heidelberg who has co-authored several new papers on the scale symmetry approach. The basic equations of particle physics need something extra to rein in the Higgs boson, and supersymmetry may not be it. Theorists like Lindner have started asking, “Is there another symmetry that could do the job, without creating this huge amount of particles we didn’t see?”

    Wrestling Ghosts

    Picking up where Bardeen left off, researchers like Salvio, Strumia and Lindner now think scale symmetry may be the best hope for explaining the small mass of the Higgs boson. “For me, doing real computations is more interesting than doing philosophy of multiverse,” said Strumia, “even if it is possible that this multiverse could be right.”

    For a scale-symmetric theory to work, it must account for both the small masses of the Standard Model and the gargantuan masses associated with gravity. In the ordinary approach to doing the calculations, both scales are put in by hand at the beginning, and when they connect in the equations, they try to even each other out. But in the new approach, both scales must arise dynamically — and separately — starting from nothing.

    “The statement that gravity might not affect the Higgs mass is very revolutionary,” Dimopoulos said.

    A theory called “agravity” (for “adimensional gravity”) developed by Salvio and Strumia may be the most concrete realization of the scale symmetry idea thus far. Agravity weaves the laws of physics at all scales into a single, cohesive picture in which the Higgs mass and the Planck mass both arise through separate dynamical effects. As detailed in June in the Journal of High-Energy Physics, agravity also offers an explanation for why the universe inflated into existence in the first place. According to the theory, scale-symmetry breaking would have caused an exponential expansion in the size of space-time during the Big Bang.

    However, the theory has what most experts consider a serious flaw: It requires the existence of strange particle-like entities called “ghosts.” Ghosts either have negative energies or negative probabilities of existing — both of which wreck havoc on the equations of the quantum world.

    “Negative probabilities rule out the probabilistic interpretation of quantum mechanics, so that’s a dreadful option,” said Kelly Stelle, a theoretical particle physicist at Imperial College, London, who first showed in 1977 that certain gravity theories give rise to ghosts. Such theories can only work, Stelle said, if the ghosts somehow decouple from the other particles and keep to themselves. “Many attempts have been made along these lines; it’s not a dead subject, just rather technical and without much joy,” he said.

    mc
    Marcela Carena, a senior scientist at Fermi National Accelerator Laboratory in Batavia, Ill.Courtesy of Marcela Carena

    Strumia and Salvio think that, given all the advantages of agravity, ghosts deserve a second chance. “When antimatter particles were first considered in equations, they seemed like negative energy,” Strumia said. “They seemed nonsense. Maybe these ghosts seem nonsense but one can find some sensible interpretation.”

    Meanwhile, other groups are crafting their own scale-symmetric theories. Lindner and colleagues have proposed a model with a new “hidden sector” of particles, while Bardeen, Lykken, Marcela Carena and Martin Bauer of Fermilab and Wolfgang Altmannshofer of the Perimeter Institute for Theoretical Physics in Waterloo, Canada, argue in an Aug. 14 paper that the scales of the Standard Model and gravity are separated as if by a phase transition. The researchers have identified a mass scale where the Higgs boson stops interacting with other particles, causing their masses to drop to zero. It is at this scale-free point that a phase change-like crossover occurs. And just as water behaves differently than ice, different sets of self-contained laws operate above and below this critical point.

    To get around the lack of scales, the new models require a calculation technique that some experts consider mathematically dubious, and in general, few will say what they really think of the whole approach. It is too different, too new. But agravity and the other scale symmetric models each predict the existence of new particles beyond the Standard Model, and so future collisions at the upgraded LHC will help test the ideas.

    In the meantime, there’s a sense of rekindling hope.

    “Maybe our mathematics is wrong,” Dine said. “If the alternative is the multiverse landscape, that is a pretty drastic step, so, sure — let’s see what else might be.

    See the full article here.

    Formerly known as Simons Science News, Quanta Magazine is an editorially independent online publication launched by the Simons Foundation to enhance public understanding of science. Why Quanta? Albert Einstein called photons “quanta of light.” Our goal is to “illuminate science.” At Quanta Magazine, scientific accuracy is every bit as important as telling a good story. All of our articles are meticulously researched, reported, edited, copy-edited and fact-checked.

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  • richardmitnick 12:38 pm on May 30, 2014 Permalink | Reply
    Tags: , , Quantum Mechanics   

    From The New York Times: “Scientists Report Finding Reliable Way to Teleport Data” 

    New York Times

    This is copyright protected material, so just a few quanta.

    MAY 29, 2014
    JOHN MARKOFF

    Scientists in the Netherlands have moved a step closer to overriding one of Albert Einstein’s most famous objections to the implications of quantum mechanics, which he described as “spooky action at a distance.”

    In a paper published on Thursday in the journal Science, physicists at the Kavli Institute of Nanoscience at the Delft University of Technology reported that they were able to reliably teleport information between two quantum bits separated by three meters, or about 10 feet.

    room
    A forest of optical elements that was part of the quantum teleportation device used by the team of physicists in the Netherlands. Credit Hanson lab@TUDelft

    Classical bits, the basic units of information in computing, can have only one of two values — either 0 or 1. But quantum bits, or qubits, can simultaneously describe many values. They hold out both the possibility of a new generation of faster computing systems and the ability to create completely secure communication networks.

    Moreover, the scientists are now closer to definitively proving Einstein wrong in his early disbelief in the notion of entanglement, in which particles separated by light-years can still appear to remain connected, with the state of one particle instantaneously affecting the state of another.

    They report that they have achieved perfectly accurate teleportation of quantum information over short distances. They are now seeking to repeat their experiment over the distance of more than a kilometer. If they are able to repeatedly show that entanglement works at this distance, it will be a definitive demonstration of the entanglement phenomenon and quantum mechanical theory.

    Read the rest here.


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  • richardmitnick 6:04 am on February 20, 2014 Permalink | Reply
    Tags: , , , , Quantum Mechanics   

    From M.I.T.: “Closing the ‘free will’ loophole” 

    February 20, 2014
    Jennifer Chu, MIT News Office

    In a paper published this week in the journal Physical Review Letters, MIT researchers propose an experiment that may close the last major loophole of Bell’s inequality — a 50-year-old theorem that, if violated by experiments, would mean that our universe is based not on the textbook laws of classical physics, but on the less-tangible probabilities of quantum mechanics.

    quasar
    Artist’s interpretation of ULAS J1120+0641, a very distant quasar.
    Image: ESO/M. Kornmesser

    Such a quantum view would allow for seemingly counterintuitive phenomena such as entanglement, in which the measurement of one particle instantly affects another, even if those entangled particles are at opposite ends of the universe. Among other things, entanglement — a quantum feature Albert Einstein skeptically referred to as “spooky action at a distance”— seems to suggest that entangled particles can affect each other instantly, faster than the speed of light.

    In 1964, physicist John Bell took on this seeming disparity between classical physics and quantum mechanics, stating that if the universe is based on classical physics, the measurement of one entangled particle should not affect the measurement of the other — a theory, known as locality, in which there is a limit to how correlated two particles can be. Bell devised a mathematical formula for locality, and presented scenarios that violated this formula, instead following predictions of quantum mechanics.

    Since then, physicists have tested Bell’s theorem by measuring the properties of entangled quantum particles in the laboratory. Essentially all of these experiments have shown that such particles are correlated more strongly than would be expected under the laws of classical physics — findings that support quantum mechanics.

    However, scientists have also identified several major loopholes in Bell’s theorem. These suggest that while the outcomes of such experiments may appear to support the predictions of quantum mechanics, they may actually reflect unknown “hidden variables” that give the illusion of a quantum outcome, but can still be explained in classical terms.

    Though two major loopholes have since been closed, a third remains; physicists refer to it as setting independence, or more provocatively, “free will.” This loophole proposes that a particle detector’s settings may “conspire” with events in the shared causal past of the detectors themselves to determine which properties of the particle to measure — a scenario that, however far-fetched, implies that a physicist running the experiment does not have complete free will in choosing each detector’s setting. Such a scenario would result in biased measurements, suggesting that two particles are correlated more than they actually are, and giving more weight to quantum mechanics than classical physics.

    “It sounds creepy, but people realized that’s a logical possibility that hasn’t been closed yet,” says MIT’s David Kaiser, the Germeshausen Professor of the History of Science and senior lecturer in the Department of Physics. “Before we make the leap to say the equations of quantum theory tell us the world is inescapably crazy and bizarre, have we closed every conceivable logical loophole, even if they may not seem plausible in the world we know today?”

    Now Kaiser, along with MIT postdoc Andrew Friedman and Jason Gallicchio of the University of Chicago, have proposed an experiment to close this third loophole by determining a particle detector’s settings using some of the oldest light in the universe: distant quasars, or galactic nuclei, which formed billions of years ago.

    The idea, essentially, is that if two quasars on opposite sides of the sky are sufficiently distant from each other, they would have been out of causal contact since the Big Bang some 14 billion years ago, with no possible means of any third party communicating with both of them since the beginning of the universe — an ideal scenario for determining each particle detector’s settings.

    As Kaiser explains it, an experiment would go something like this: A laboratory setup would consist of a particle generator, such as a radioactive atom that spits out pairs of entangled particles. One detector measures a property of particle A, while another detector does the same for particle B. A split second after the particles are generated, but just before the detectors are set, scientists would use telescopic observations of distant quasars to determine which properties each detector will measure of a respective particle. In other words, quasar A determines the settings to detect particle A, and quasar B sets the detector for particle B.

    The researchers reason that since each detector’s setting is determined by sources that have had no communication or shared history since the beginning of the universe, it would be virtually impossible for these detectors to “conspire” with anything in their shared past to give a biased measurement; the experimental setup could therefore close the “free will” loophole. If, after multiple measurements with this experimental setup, scientists found that the measurements of the particles were correlated more than predicted by the laws of classical physics, Kaiser says, then the universe as we see it must be based instead on quantum mechanics.

    “I think it’s fair to say this [loophole] is the final frontier, logically speaking, that stands between this enormously impressive accumulated experimental evidence and the interpretation of that evidence saying the world is governed by quantum mechanics,” Kaiser says.

    Now that the researchers have put forth an experimental approach, they hope that others will perform actual experiments, using observations of distant quasars.

    Physicist Michael Hall says that while the idea of using light from distant sources like quasars is not a new one, the group’s paper illustrates the first detailed analysis of how such an experiment could be carried out in practice, using current technology.

    “It is therefore a big step to closing the loophole once and for all,” says Hall, a research fellow in the Centre for Quantum Dynamics at Griffith University in Australia. “I am sure there will be strong interest in conducting such an experiment, which combines cosmic distances with microscopic quantum effects — and most likely involving an unusual collaboration between quantum physicists and astronomers.”

    “At first, we didn’t know if our setup would require constellations of futuristic space satellites, or 1,000-meter telescopes on the dark side of the moon,” Friedman says. “So we were naturally delighted when we discovered, much to our surprise, that our experiment was both feasible in the real world with present technology, and interesting enough to our experimentalist collaborators who actually want to make it happen in the next few years.”

    Adds Kaiser, “We’ve said, ‘Let’s go for broke — let’s use the history of the cosmos since the Big Bang, darn it.’ And it is very exciting that it’s actually feasible.”

    This research was funded by the National Science Foundation.

    See the full article here.


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  • richardmitnick 1:05 pm on January 24, 2014 Permalink | Reply
    Tags: , , , Quantum Mechanics   

    From Fermilab: “Physics in a Nutshell “Nobody understands quantum mechanics” 


    Fermilab is an enduring source of strength for the US contribution to scientific research world wide.

    Friday, Jan. 24, 2014
    Jim Pivarski

    Of all the scientific theories that have broken out into public consciousness, none have ranged as far as quantum mechanics. This subject is sometimes presented as an erudite abstraction, as a smokescreen of uncertainty, as an almost mystical philosophy or as evidence that physicists have lost their minds. It’s rarely said that quantum mechanics makes sense.

    switch
    Intuitively, we expect physical quantities such as energy to be continuous and single-valued, like a dimmer switch. At very small scales, however, they are both discrete and multi-valued, like a light switch that can be on and off at the same time.

    There is good reason for that. Quantum mechanics is as hard to believe as anything can be while being demonstrably true. [Richard] Feynman’s famous quote, “I think I can safely say that nobody understands quantum mechanics,” is sometimes taken out of context as suggesting that if you think you get it, you don’t. This defeatist attitude is unnecessary. Quantum mechanics is bizarre, but it can be understood.

    The rules of quantum mechanics are logical, yet unfamiliar. For example, we expect a physical quantity like the position of a particle to be a single number, something that could be measured by a ruler. It is here and not there. That number may vary continuously as the particle moves, and it may be imprecisely known if we have not measured it well, but we intuitively expect it to be a specific number at a specific time.

    What physicists have learned is that the position of a particle is not a single number: It is multi-valued. The particle is here and there in a way that can be quantified, called the wavefunction. We imagine the wavefunction as a blob filling space, describing the degree to which the particle is in each place: thicker here, thinner there. It can be measured and charted, but our brains don’t like it because we evolved to manipulate the macroscopic world, everything larger than a splinter and smaller than a mammoth. Studying quantum mechanics forces us beyond our comfort zone, to apprehend something truly alien and shed our macrocentrism.

    When I first learned about quantum mechanics, I was bothered by the crispness of quantum properties almost as much as their fuzziness. Not only is the energy of a particle multi-valued, but each of those values is a whole number, never a fraction. It is as though the sliding dimmer light of our intuition has been replaced by an on-off switch with no middle value, but one that can be 30 percent on and 70 percent off, or any other ratio. Quantities have surprisingly little freedom in what values they can take, but surprisingly much freedom in how many they can take at once.

    This is the first in a four-part series on quantum mechanics. In the next article, I will present the time paradoxes, followed by wave-particle duality and an overview of how we know what we know.

    See the full article here.

    Fermilab Campus

    Fermi National Accelerator Laboratory (Fermilab), located just outside Batavia, Illinois, near Chicago, is a US Department of Energy national laboratory specializing in high-energy particle physics.


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